The present invention relates generally to filtering a signal read from a data storage medium and, more particularly, to a system and method for calibrating a corner frequency of a tunable filter used for filtering a signal read from a data storage medium.
Tunable filters, such as integrated continuous time filters (CTFs), are known to be particularly useful when implemented in the read channel of a disk drive system for purposes of filtering readback signals obtained from a data storage disk. A typical disk drive system includes a magnetic medium for storing data in magnetic form and a number of transducers used to write and read magnetic data respectively to and from the medium. Digital information is typically stored in the form of magnetic transitions on a series of concentric, spaced tracks formatted on the surface of the magnetizable rigid data storage disks.
Reading data from a specified disk location is typically accomplished by using a read element of the read/write head assembly to sense the magnetic field or flux lines emanating from the magnetized locations of the disk. As the read element passes over the rotating disk surface, the interaction between the read element and the magnetized locations on the disk surface results in the production of electrical signals, commonly referred to as readback signals, in the read element.
A filter employed in a read channel must generally be capable of accommodating variations in the data rate of the readback signals transmitted through the read channel. The ability to precisely tune such a filter is therefore of particular importance. It is generally considered imperative that a cutoff or corner frequency of a tunable filter used in such applications be adjustable with a high degree of precision. Such applications may also require that the tunable filter be adjustable within a range of selectable cutoff frequencies with high precision.
It is well understood in the art that tunable filters implemented in integrated circuits, such as VLSI chips for example, exhibit frequency response characteristics that are sensitive to technology and process variations. Technology variations, as well as factors resulting from circuit/filter design or implementation, often result in large tolerances with respect to the filter""s specified cutoff frequency. Such tolerances can result in performance losses.
There exists a need for an apparatus and method for precisely adjusting the cutoff frequency of a tunable filter to a specified frequency or frequencies. There exists a need in the disk drive system manufacturing community for such an apparatus and method suitable for implementation in-situ a disk drive system and, more particularly, in-situ a read channel of the disk drive system. The present invention fulfills these and other needs.
The present invention is directed to a system and method for calibrating a corner frequency of a tunable filter coupled to a read channel of a data storage system to a desired frequency. A corner frequency calibration method according to the present invention involves generating a signal having a frequency equal to the target corner frequency and adjusting a gain of the signal to a gain threshold. The gain of the signal is adjusted from the gain threshold by a pre-established amount, such as by 3 dB.
The corner frequency of the signal is shifted until the gain of the signal is substantially equal to the gain threshold, at which point the corner frequency of the tunable filter is calibrated to the desired frequency. The calibrated corner frequency of the tunable filter may be a low or high corner frequency appropriate for filtering a data signal or a servo signal.
Shifting the corner frequency of the signal typically involves shifting the corner frequency of the signal downward in frequency. Shifting the corner frequency of the signal also involves shifting the corner frequency of the signal while the gain of the signal is held at a gain level equal to the gain threshold plus the pre-established gain amount.
The method may further involve converting the signal from analog form to digital form to produce a digital signal. In this case, adjusting the gain of the signal involves digitally adjusting the gain of the digital signal to a digital gain threshold. Increasing the gain of the digital signal involves incrementally increasing the digital signal gain. Generating the signal, according to one embodiment, involves generating a peak-detectable signal, such as a triangular or sinusoidal signal. The signal may be an ECL (emitter-coupled-logic) signal or CMOS (complimentary metal oxide semiconductor) signal, for example.
A circuit for calibrating a cutoff frequency of a filter to a desired frequency includes a tunable filter and a variable gain amplifier coupled to the filter and a signal source. The signal source generates a signal used in the corner frequency calibration procedure. An analog-to-digital converter (ADC) is coupled to the filter. The ADC produces a digitized signal in response to receiving the signal from the filter.
A control circuit is coupled to the filter, amplifier, signal source, and ADC. The control circuit adjusts a gain of the digitized signal to a gain threshold, increases the gain of the digitized signal from the gain threshold by a pre-established amount, and shifts the corner frequency of the digitized signal until the gain of the digitized signal is substantially equal to the gain threshold, at which point the corner frequency of the filter is calibrated to the desired frequency.
The signal source, according to one embodiment, includes a signal translator. The signal translator communicates a signal, such as an ECL signal, generated by the signal source to the amplifier. The signal source may further include a sequencer coupled to the signal translator. The sequencer includes servo inputs coupled to a servo voltage controlled oscillator and data inputs coupled to a tunable write voltage controlled oscillator.
The control circuit communicates control signals to the signal source to set the signal source to a frequency to be divided into the desired corner frequency. The control circuit comprises a digital peak detector circuit that digitally peak detects the gain of the digitized signal received from the ADC. The control circuit communicates a control signal to the ADC to set a sampling frequency of the ADC. ADC capture logic may be coupled between the ADC and the control circuit. A digital gain controller is typically coupled between the control circuit and the amplifier. The digital gain controller provides amplifier control signals to the amplifier in response to control signals received from the control circuit.
The filter may be implemented as a tunable lowpass filter, tunable highpass filter or tunable bandpass filter. The corner frequency calibration circuitry may be implemented in-situ a read channel of a disk drive system.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.